Zener diodes are bipolar semiconductor devices that have a p-n junction interface with high carrier densities. They utilize breakdown (avalanche breakdown or Zener breakdown) caused when a reverse voltage is applied. When the breakdown voltage is caused, the diodes keep the voltage between terminals constant irrespective of the current (hereinafter, the Zener voltage).
Zener diodes using Si or the like are known in the art. The present inventors then studied methods for manufacturing Zener diodes using silicon carbide (SiC). Compared to silicon (Si), SiC has superior properties such as approximately ten fold dielectric breakdown strength and approximately three fold thermal conductivity, and attracts attention as a material suited for power semiconductor devices.
Known SiC bipolar semiconductor devices include p-n diodes (Patent Document 1, Non-Patent Documents 1 to 7). For example, compared to Si p-n diodes that withstand 10 kV, SiC p-n diodes withstanding such a high voltage have a low forward voltage, being approximately ⅓, and an high speed-reverse recovery time corresponding to the speed during the OFF operation, being approximately 1/20 or less, and can reduce electric loss to approximately ⅕ or less, thereby greatly contributing to energy saving.
SiC bipolar devices other than SiC p-n diodes, for example SiC n-p-n transistors, SiC SIAFET and SiC SIJFET are also reported to reduce electric loss.    Patent Document 1: JP-A-2002-185015    Non-Patent Document 1: Material Science Forum Vols. 389-393 (2002), pp. 1317-1320    Non-Patent Document 2: Material Science Forum Vols. 457-460 (2004), pp. 1029-1032    Non-Patent Document 3: SiC Kei Seramikku Shinzairyou (SiC ceramic new materials) (Japan Society for the Promotion of Science, 124th Committee, UCHIDA ROKAKUHO PUBLISHING CO., LTD., P. 342)    Non-Patent Document 4: Journal of Applied Physics, Vol. 96, No. 9 (2004), pp. 4916-4922    Non-Patent Document 5: Journal of Applied Physics, Vol. 92, No. 10 (2002), pp. 5863-587    Non-Patent Document 6: Journal of Applied Physics, Vol. 92, No. 1 (2002), pp. 549-554    Non-Patent Document 7: Journal of Applied Physics, Vol. 86, No. 2 (1999), pp. 752-758
In the manufacturing of SiC Zener diodes, a p-n junction is formed by epitaxial growth of an SIC conductive layer of a first conductivity type on an SiC substrate of a first conductivity type, and then epitaxial growth of an SiC conductive layer of a second conductivity type on the surface of the SIC conductive layer of a first conductivity type or ion-implantation of a second conductivity type into the surface of the SiC conductive layer of a first conductivity type.
FIG. 2 shows relations between carrier densities and Zener voltage of SiC Zener diodes manufactured as described above wherein the p-n junction is a step junction and the acceptor density≧the donor density. The voltage values indicated in the figure are Zener voltages. The figure shows that diodes having a wide range of Zener voltages may be obtained by appropriately selecting the donor density and the acceptor density. It will be understood that the carrier densities should be high in order to obtain diodes having a low Zener voltage.
The upper limits of the carrier densities are restricted by, for example, solid solubility limit of dopants (also referred to as impurities) in SiC. For example, Non-Patent Documents 3 and 4 report that aluminum which is a dopant in a p-type conductive layer has a solid solubility limit in SiC of approximately 2×1021 cm−3, and that the upper limit of solid solubility for aluminum to be able to function as acceptor in SiC is approximately 8.9×1019 cm−3. Similarly, the upper limit of solid solubility is approximately 4×1019 cm−3 for nitrogen which is a dopant in an n-type conductive layer to be able to function as donor in SiC (Non-Patent Documents 6 and 7). It is reported that stacking fault becomes marked in a nitrogen-doped n-type conductive layer when the donor density is 2×1019 cm−3 or more (Non-Patent Document 5).
SiC Zener diodes manufactured by the conventional methods have problems as described below.
First, epitaxial growth of an SiC conductive layer of a second conductivity type encounters the following problem. In the process of epitaxially growing an SiC conductive layer, the doping density is unstable during the transition period from immediately after the epitaxial growth is initiated (the gas introduction is started) until a steady state is reached.
An exemplary diode production case will be considered wherein an n-type conductive layer is formed on an n-type substrate and a p-type conductive layer is epitaxially grown thereon and wherein the n-type conductive layer has a donor density of 1×1018 cm−3 and the target Zener voltage of the diode is 70 V. In this case, the p-type epitaxial layer should have an acceptor density of 1×1019 cm−3. However, as shown in FIG. 3, an acceptor density in the vicinity of the p-n junction that deviates from the designed value changes the obtainable Zener voltage ranging from 60 V to 130 V. As a result, the Zener voltage cannot be obtained as designed and yields are deteriorated. In particular, it is illustrated that a Zener voltage that greatly deviates from the designed value will result when the acceptor density is lower than designed.
The allowable transition period is generally estimated from the width of the depletion layer extending in the p-type conductive layer and the epitaxial growth rate. FIG. 4 shows relations between the Zener voltage and the width of a depletion layer extending in a p-type conductive layer (namely, the thickness of a depletion layer formed in a p-type conductive layer).
From FIG. 4, diodes with a Zener voltage of 10 V to 100 V have a depletion layer extending in a thickness of several nm to about 100 nm in the p-type conductive layer. That is, the acceptor density in the p-type conductive layer should be stabilized at least while the p-type conductive layer is grown from the p-n interface to a thickness of several hundreds of pm to 10 nm (approximately 1/10 of the thickness of the depletion layer). If the assumption is made that the epitaxial growth rate is 2 to 20 μm/h, the acceptor density has to be stabilized within several seconds after the initiation of the epitaxial growth. However, current epitaxial growing technology is incapable of such control.
Non-Patent Document 2 reports mesa SiC Zener diodes having a Zener voltage of about 22 V. The diodes are described to have an acceptor density of 1×1019 cm−3. From FIG. 2, however, the Zener voltage obtainable with that acceptor density will be likely in the range of 40 to 50 V. This Zener voltage of approximately 22 V is considered to have caused by conduction due to an electric field locally concentrated at mesa ends or by a higher-than-expected acceptor density in the p-type conductive layer in the vicinity of the p-n junction interface.
As described above, obtaining a Zener voltage as designed and with reproducibility is difficult when a p-n junction is formed by epitaxial growth.
Ion implantation is an alternative to epitaxial growth to produce p-type conductive layers. The ion implantation permits relatively precise control of the doping density compared to the epitaxial growing. The Zener voltage of diodes having a p-type conductive layer by ion implantation is dependent on the donor density in the n-type conductive layer and ion implanting conditions (e.g., ion species, dose, implanting energy).
FIG. 5 shows relations among ion implanting energy, Zener voltage and implantation depth in the production of p-type conductive layers wherein the p-type conductive layers are formed on respective n-type conductive layers having a donor density (Nd) of 1×1017 cm−3, 1×1018 cm−3, 2×1018 cm−3, 2×1019 cm−3 or 4×1019 cm−3 with use of aluminum as dopant such that the doping density of the p-type conductive layer from the ion implantation surface to the ion implantation depth will be 2×1021. The Zener voltage is shown to increase with increasing implanting energy or increasing implantation depth in the range of implanting energy from 10 to 500 keV. The figure also shows a trend that the relations between the Zener voltage and the other two reach saturation at implanting energy of above 500 keV.
In practical diodes, an anode electrode is formed on the surface of a p-type conductive layer. A thin metal layer as an electrode material is formed on the surface of a p-type conductive layer, and the electrode material metal and SiC are annealed at high temperatures to form an alloy layer, resulting in an ohmic electrode. The distance between the alloy layer and the p-n junction interface, namely the thickness of the p-type conductive layer should be large enough to prevent punching through. For example, the thickness of the p-type conductive layer is 1 μm or more. When a p-type conductive layer is formed by ion implantation as described above, it is difficult as shown in FIG. 5 that the diode has a Zener voltage of 30 to 40 V or below even if conductive layers with high carrier densities are adopted.
To form a p-type conductive layer in a thickness of 1 μm or more by ion implantation, implanting energy of 1 MeV or more is required. Provided that the maximum implanting energy is 1 MeV and a box profile is formed which has an Al doping density of 2×1021 cm−3 corresponding to the solid solubility limit of Al, the dose of Al ions is approximately 2×1017 cm−2. Ion implantation involving such high dose energy and high dose increases costs, and therefore it is not preferable that p-type conductive layers are formed by ion implantation alone.
The present invention has been made in order to solve the problems in the background art as described above. It is therefore an object of the invention to provide bipolar semiconductor devices that have a Zener voltage controlled highly precisely in a wide range of Zener voltages (for example, from 10 to 500 V). It is another object of the invention to manufacture diodes having a wide range of Zener voltages (for example, from 10 to 500 V) with high yields.
Attempts to produce Zener diodes having a wide range of Zener voltages (for example, from 10 to 500 V) with high yields by conventional methods have encountered the problems summarized as follows.
When Layer of a Second Conductivity Type is Formed by Epitaxial Growing:
The carrier densities should be stabilized within several seconds after the initiation of the epitaxial growth. However, the current epitaxial growing technology is incapable of such control.
When Layer of a Second Conductivity Type is Formed by Ion Implantation Alone
The layer of second conductivity type should be formed in a thickness of 1 μm or more, namely, with implanting energy of 1 MeV or more, to prevent punching through. Such ion implantation process is not preferable because it involves high energy and high dose and increases costs. Further, this technique cannot produce Zener diodes having a Zener voltage of 40 V or below.